1. Field of the Invention
This invention relates to the field of timing signal generation and regeneration for digital transmission where an external frequency is used and in particular to providing clocking signals in connection with digital format data transmission for digital signals having only data bits and having no framing reference signals defining the start and end of a data bundle.
2. Background Art
In a typical digital data transfer, digital data is multiplexed into a serial stream and transmitted a demultiplexer for recovery and decoding. For accurate data recovery, it is necessary to define data "segments" or "windows" to enable the demultiplexing circuitry to determine when one segment begins and another one ends. This permits the demultiplexing circuitry to look for a "zero" or a "one" bit at the proper moment in time. If the demultiplexing circuitry looks for bit information at the wrong time, an erroneous reading will result.
In prior art data transfer applications, a window is defined by providing a signature bit or bits for use as a "frame signal". The signature bit sequence of frame bits do not represent any data but rather define a boundary of a data frame. Data transfers made in this method are known as time division multiplexed (TDM) bus transfers.
One such prior art scheme is disclosed in Rodman et al, U.S. Pat. No. 4,411,007. Rodman is directed to a TDM communication system incorporating delays in the transmission so that a packet arrives at a central station within the right time frame regardless of the transmitters distance from the central station. Shift registers are used to implement the delay in the transmission. Rodman is directed to a "DS1" digital data format. The DS1 digital format supplies a framing bit to identify data segments.
Troost, U.S. Pat. No. 4,488,292 is directed to a TDM network utilizing delays between frames and compensating for the effects of line delays by using a memory to create delays approximating a full frame. The method of Troost requires and relies on the presence of a frame mark bit defining individual pulse frames.
Certain data transmission formats lack the frame bit required by Rodman and Troost. For example, a "DS0" data transfer format lacks the frame bit of the DS1 format. In the DS0 format, demultiplexing circuitry utilizes the bit stream itself to provide a clocking signal which may be used for frame identification.
In the DSO format, data arrives at a multiplexer clocked from a remote site with a clock that is nominally the same frequency as the clock derived at the site. This data provides the bit "1" time position. Such transmission requires an external clock to define boundaries of data segments. Because of transmission delays of clock to mux and data from mux to mux, limits exist for cross connecting data based on a composite of the clock to mux distance and mux to mux distance.
In a digital network, various devices, such as channel banks, digital switches, ISDN hardware, and other devices require synchronized distribution timing signals at the DS0 or DS1 data rate. When in the DS1 data format, only frequency synchronization is necessary because phase information is contained in the data stream itself. In the DS0 data format, both phase and frequency synchronization is required.
Therefore it is an object of the present invention to provide circuitry allowing DS0 format data transfers of maximum length and provide phase adjustment over greater length data transfers.
It is yet another object of the present invention to provide circuitry for correcting phase shift problems over extended length transmission lines.